Platelet-derived growth factors (PDGFs) are a family of potent mitogens for almost all mesenchyme-derived cells. There are four PDGF isoforms, A, B, C and D, that form five different disulphide-linked dimeric proteins PDGF-AA, -BB, -AB, -CC, and -DD. These growth factors exert their cellular effects through two structurally related tyrosine kinase receptors: PDGF receptor α (PDGFRα) and PDGF receptor β (PDGFRβ). (Sandy, J. R., 1998, Br. J. Orthod. 25:269-74; Betsholtz, C. et al., 2001, Bioessays 23:494-507).
PDGFRα and PDGFRβ are structurally similar and can form heterodimers as well as homodimers. PDGF-BB and PDGF-DD are the primary activators of ββ homodimeric receptors. PDGF-AA activates only αα receptor dimers, whereas PDGF-AB, PDGF-BB, and PDGF-CC activate αα and αβ receptor dimers. The dimeric ligand molecules bind to two receptor proteins simultaneously and induce receptor dimerization, autophosphorylation of specific residues within the receptor's cytoplasmic domain, and intracellular signaling. Ultimately, the activation of PDGFRβ signaling pathway induces various cellular responses, including cell proliferation and migration.
Angiogenesis is believed to be essential for both tumor growth and metastasis. Development of a vascular system, or vasculogenesis, involves the assembly of two principal cell types, endothelial cells (EC) and pericytes/vascular smooth muscle cells (SMC), into mature blood vessels. The involvement of the PDGF-B/PDGFRβ signaling pathway in vasculogenesis is suggested by PDGF-B and PDGFRβ knockout mice. The PDGF-B and PDGFRβ knockout phenotypes are virtually identical in that mice exhibit hemorrhages due to the loss of coverage of pericytes and smooth muscle cells in vessels. The studies indicated that PDGFRβ is involved in pericyte recruitment to capillaries and development of smooth muscle cells in vessels. Pericyte recruitment to coat nascent vessels is essential for the stabilization and further establishment of the vascular network. Vessels lacking adequate pericyte coverage are more vulnerable to VEGF inhibition than well-established mature vessels.
A number of tyrosine kinase inhibitors under development as anti-tumor agents have been found to inhibit PDGFRβ. However, these compounds have multiple tyrosine kinase targets. For example, imatinib mesylate (Gleevec®/ST571) was developed as an Abelson (Abl) tyrosine kinase inhibitor, and also inhibits c-kit, PDGFRα, and PDGFRβ. Sunitinib malate (Sutent®/SU11248) is a broad-spectrum, orally available multitargeted tyrosine kinase inhibitor with activity against VEGFR, PDGFR, c-KIT, and FLT-3. CP-673,451 is an inhibitor of both PDGFRα and PDGFRβ. Since these small molecule antagonists are not specific to these receptors, it is not possible to distinguish the contribution of PDGFRβ signaling to angiogenesis, including tumor-associated angiogenesis, tumor stimulation and growth, or toxicities associated with administration of such compounds that might be due to unnecessary targeting of multiple receptors.
Accordingly, the invention provides PDGFRβ-specific antagonists, demonstrates the role of PDGFRβ in angiogenesis, including angiogenesis that supports tumor growth and survival, and provides methods of treating tumors and angiogenic diseases. The invention further demonstrates the advantage of simultaneously inhibiting signal transduction through PDGFRβ and VEGFR.